The millimeter-wave (MM-wave) region of the electromagnetic spectrum (defined herein to be radiation at wavelengths less than about 10 millimeters and having a frequency greater than about 30 GHz) is becoming increasingly important for many commercial and military applications. This is due to the lack of frequencies for new services in the microwave region of the spectrum, and also to the advantages to be gained from operation at higher frequencies. Furthermore, the MM-wave region of the electromagnetic spectrum is at present relatively uncrowded so that wider bandwidths are available for signal transmission, and there is reduced signal interference. A number of other advantages are to be gained by operating at MM wavelengths. The use of MM-waves allows transmitted signal beams to be more directive in nature, and also provides a greater image resolution for radar or ranging applications. Additionally, the use of MM-waves allows the use of reduced transmitter power levels and smaller antenna sizes. As a result, system size and weight can be greatly reduced compared to equivalent microwave systems. Furthermore, transmission at MM-wave frequencies is improved compared with microwave systems due to a relative immunity to weather conditions. At MM-wave frequencies, the transmitted data can be greatly increased, increasing the channel capacity for communications and thereby lowering network system cost. And for many MM-wave applications, the directionality and limited range of signal transmission means that the same frequency bands can be allocated to many different localities without interference.
MM-wave technology has both commercial and military applications. The near-term applications are primarily in the areas of radar and communications; although very-high-speed signal processing and computing are expected to become of increasing importance as millimeter-wave monolithic integrated circuit (MMIC) and opto-electronic integrated circuit (OEIC) technology is developed. Areas of commercial application include data and signal processing, intelligent automotive systems for traffic control and safety; imaging radar systems for aircraft landing under low visibility conditions; vision for autonomous robotic vehicles; intrusion alarms; local-area and satellite direct broadcast systems for computer networks and personal communications; and fiber-optic communication links. Military applications include radar fuses for projectiles; "smart" extended-range weapons; adverse-weather weapon systems; electronic warfare; phased-array radar; global positioning systems; and defense satellite communications.
The development of new and improved sources of MM-wave radiation is essential to the development and application of the technologies listed above and to he development of future MMIC and OEIC technology. The most commonly used solid-state electrical sources of MM-wave radiation today are negative differential resistance diodes such as Gunn and IMPATT diodes. IMPATT diodes have been the most powerful and efficient electrical MM-wave devices for the frequency range of 50to 100 GHz. With increasing frequency above 50 GHz, however, the output power of IMPATT diodes decreases very rapidly due to a saturation of the ionization rate at high electric fields. Multi-quantum-well structures are being developed for IMPATT diodes to reduce this limitation; but these devices are presently limited to continuous-wave (cw) output powers of less than about 10 milliwatts at 100 GHz.
An advantage of the present invention is that an apparatus for generating a MM-wave electrical signal can be formed as a part of an OEIC or a MMIC wherein a mode-locked lasing pulse train at a frequency .gtoreq.10 GHz is generated by a semiconductor ring laser and converted by a high-speed photodetector into an electrical output signal at the same frequency.
Another advantage of the present invention is that a frequency of the electrical signal to be generated can be controlled by providing a particular size for a semiconductor ring laser used to generate the mode-locked lasing pulse train.
A further advantage is that electrical feedback from the high-speed photodetector can be used to reduce any optical jitter in the mode-locked lasing pulse train to stabilize a frequency of oscillation of the electrical output signal from the high-speed photodetector.
Yet another advantage of the present invention is that an amplitude of the generated electrical signal can be controlled or increased by a optical waveguide amplifier located between the semiconductor ring laser and the high-speed photodetector and optically coupling the laser and photodetector.
Yet another advantage is that the apparatus of the present invention can be operated to generate an electrical signal at a frequency that is twice the frequency of the mode-locked lasing pulse train within the semiconductor ring laser.
These and other advantages of the apparatus of the present invention will become evident to those skilled in the art.